See screenshots; the "block" in front is a custom model on an entity's head. This works fine in 1.21.1.

Normal glass is unaffected. (The resource pack in initial screenshots makes white stained glass look like normal glass.)

Steps to reproduce

1. Enable the attached resource pack.
2. Use the commands to spawn the model and place light gray stained glass (invisible) in it:

   /execute align xyz run summon armor_stand ~0.5 ~5.6 ~0.5 {ArmorItems:[{},{},{},{id:"diamond_pickaxe",count:1}],NoGravity:1b}
   /fill ~ ~1 ~ ~ ~3 ~ light_gray_stined_glass

3. Place white stained glass nearby.
4. Look through the model in the direction of the stained glass.
5. Change your graphics settings.

Observed results

The white stained glass will bleed through the punching bag model, on fabulous graphics.

Expected results

It would render the same as in fast or fancy graphics, the punching bag being fully in front.

Notes

Breaking the stained glass in the model corrects the rendering, and disabling the resource pack shows that stained glass on stained glass renders fine. All 3 elements are needed to see this issue.